Profibus is an industrial network which can have a constant time cycle. Isochronous I/O over Profibus, also referred to as equidistance or constant bus cycle time, requires internal timing specifications within a specified error rate (jitter). If the timing is not maintained within this proscribed error rate, I/O is negatively impacted. It will be appreciated that other industrial networks are adding the capability of a constant time cycle. Examples of other networks include DeviceNet & EthernetIp.
Conventional systems and methods that may achieve this proscribed error rate are both expensive and inefficient. For example, isochronous I/O provides an interrupt to the I/O module that is the timing reference for all synchronous activity. Thus, in one conventional system and method, a PLL could be implemented to track this periodic interrupt and to multiply it by a large factor to provide the timing for module samples. This method is an expensive way to achieve the timing requirements. In a second conventional system and method, a crystal oscillator could be used to provide a tighter tolerance time base for the module to meet these timing requirements. However, it is unacceptably expensive.
Therefore, there is a need for a system and method that efficiently and inexpensively meets the proscribed error rate of the internal timing specifications of isochronous I/O over Profibus.